U. 7

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Digestive system
Nutrition: It's a process by which a living organism ingests food, digest it, absorb
it, assimilate it, and lastly egest it.
Ingest
Digest
Absorb
Assimilate
Egest
Ingestion: The intake of any substance (e.g.: Food or Drinks), through
the mouth.
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Biology / Chapter 7
Digestion
Mechanical
Mechanical
Breakdown of food, without
chemically changing it.
Chemical
Chemical
Breakdown of partially digested
food, and insoluble food.
Chewing, grinding in mouth
(Teeth, Tongue).
Churning, mixing in stomach.
Using enzymes and juices
Peristalsis in all body parts,
especially the Oesophagus.
In mouth (salivary amylase)
In stomach (Gastric Juices, HCl)
In small intestine (Pancreatic
juices and bile)
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Biology / Chapter 7
Absorption
Movement of the small food
molecules and ions
Through the walls of the small
intestine into the blood.
Assimilation
Movement of digested food
molecules
Into the cells of the body,
where they're used
Therefore becoming part of
the cell
Egestion (Elimination, Excretion)
Excreting food that has
not been digested or
absorbed.
As faeces through the
anus
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Biology / Chapter 7
Alimentary Canal
Structure
Mouth
Salivary glands
Oesophagus
Function
Where food enters the alimentary canal
and digestion begins
Produce saliva containing amylase
Muscular tube which moves ingested
food to the stomach
Stomach
Muscular organ where digestion
continues
Pancreas
Produces digestive enzymes
Liver
Gall bladder
Produces bile
Stores bile before releasing it into the
duodenum
Small intestine duodenum
Where food is mixed with digestive
enzymes and bile
Small intestine ileum
Where digested food is absorbed into the
blood and lymph
Large intestine colon
Where water is reabsorbed
Large intestine rectum
Where faeces are stored
Large intestine - anus Where faeces leave the alimentary canal
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Mouth (Buccal Cavity)
Consists of teeth and tongue
Digestion
Mechanical
The teeth cut, tear, grind, and
crush the food, increasing its
surface area.
Chemical
StarchSalivary Amylase Maltose sugar
Happens at pH (7-7.5), which is
alkaline
Tongue mixes the food with the
saliva forming it into a bolus.
Then it's swallowed.
Salivary amylase
Saliva
Water
Mucus
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Biology / Chapter 7
•
Amylase catalyses: starch → maltose
•
Maltase catalyses: maltose → glucose
The digestion of starch to
glucose needs two enzymes
Teeth:
Contribute in the ingestion and mechanical digestion of food.
Teeth Structure:
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Biology / Chapter 7
Enamel
The hard outer layer of the
crown to protect dentine
(Hardest substance in the
body).
Can be dissolved by acids
Non-living outer layer
Living layer with channels of
cytoplasm.
Dentine
Hard but not as hard as enamel.
Forms the bulk of the tooth
Can be sensitive if enamel is
lost.
Contain nerves for sensitivity
Pulp cavity
Contain blood vessels for food
and oxygen supply.
Contain cells which divide to
make dentine
Extends from the crown to the
tip of the root
Bone like substance.
Cementum
Has fibres growing out of it to
attach the tooth to the jawbone
and to allow the tooth to move
slightly when biting.
Covering the root
Not as hard as enamel
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Types of human teeth
Incisor
•
•
•
Canine
Molar & Premolar
Incisor – for biting and cutting
Canine – for holding and cutting
Premolar and Molar - crushing and chewing
Difference between carnivorous and herbivorous
teeth
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Herbivorous
Carnivorous
Have teeth that are highly specialized
for eating plants; because plant matter
(cellulose) is often difficult to break
down,
Have teeth that are very different from
herbivores'; because they have a
different diet.
Their molars are wider and flatter,
designed to grind food, and aid in
digestion.
Their incisors are sharp for tearing
plants, but they may not be present on
both the upper and lower jaw.
E.G.: White tail deer (has only lower
incisors and a rigid upper jaw).
Horses and cows have jaws that are
capable of moving sideways.
•
Use its teeth to kill a prey item before
eating it.
The sharp incisors and pointed canine
teeth are designed for both
incapacitating and eating a meal
.
Canine tooth can be easily identified,
as it is the longer, pointed tooth
located on either side of the incisors.
The molars are fewer in number;
because so much of the work is done
by the teeth in the front of the mouth.
E.G.: Tigers, Lions, Cheetahs,
Hyenas, Cougars, Foxes, Mountain
lions, Coyotes, Hawks.
During the lifecycle of a mammal he has 2 sets of teeth, milk teeth (deciduous teeth) which
are 20, later on in his life they're replaced by permanent teeth which are 32, equally
distributed in each jaw.
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Tooth Decay:
•
Tooth decay happens when the hard outer enamel of the tooth is damaged. This can happen
when bacteria in the mouth convert sugars into acids that react with the enamel. Bacteria can
then enter the softer dentine inside.
Tooth decay can be prevented by:
•
Avoiding foods with a high sugar content.
•
Using toothpaste and drinking water containing fluoride.
•
Regular, effective brushing to prevent the build-up of plaque (a sticky layer on the teeth), which
is an average of 3 times per day.
•
Dental check-ups.
•
Eat a balanced diet that contains Calcium and Vitamin D.
Fluoride compounds may be added to toothpaste and public water supplies.
Fluoride reduces tooth decay by:
•
•
Reducing the ability of bacteria on plaque to produce acid.
Helping to replace calcium ions and phosphate ions lost by tooth enamel because of acid
attack.
However, there are arguments against fluoridation of drinking water. For
example:
•
Some people say that they should not be forced to consume fluoride.
•
Excessive fluoride can cause grey or brown spots on teeth.
The Pharynx:
•
•
Common passage between the respiratory and digestive systems.
•
The epiglottis; a flap of cartilage; stops the food from going down into the lungs and closes
the end of trachea.
Peristalsis: It's a process by which food is moved through the digestive system, by the help
of two muscles in the gut wall
1. Circular muscles - which reduce the diameter of the gut when they contract.
2. Longitudinal muscles - which reduce the length of the gut when they contract.
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The muscles work together to produce wave-like contractions. These have a ‘squeezing
action’ that pushes the bolus through the gut.
Oesophagus: It's a muscular tube which transports food from the mouth to the stomach, by
peristalsis.
The stomach:
•
A muscular sac-like organ located on the left side of the upper abdomen, and is the widest
organ in the alimentary canal.
•
The stomach muscles contract periodically, churning food to enhance digestion, while
simultaneously secreting gastric juices including hydrochloric acid, pepsinogen, and mucus
partially digesting fats and proteins, and changing them into chyme.
•
Hydrochloric acid is used for:
1. Killing bacteria
2. Provides the optimum pH for inactive
pepsinogen to change into active pepsin.
•
Protein By Pepsin
(Happens at pH=2)
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Polypeptides
Biology / Chapter 7
•
G.R.F: The stomach doesn't digest itself.
1. Pepsin is secreted in its inactive form (Pepsinogen)
2. Stomach secrets a protective layer of mucus from the goblet cells.
The Small Intestine
Duodenum
•
Jejunum
ileum
The small intestine is about (6.7 - 7.6m).
Duodenum:
•
•
•
•
The smallest part of the small intestine (23 – 28cm)
Responsible for the absorption of nutrients from the already digested food.
After food is digested in the stomach, it reaches the duodenum through pyloric sphincter.
Ducts from the liver, gallbladder, and pancreas enter the duodenum to provide juices that
neutralize acids coming from the stomach and help digest proteins, carbohydrates, and fats.
Enzyme
Substrate
End-products
Where produced
Salivary amylase
Starch
Maltose
Salivary glands
Pepsin
Lipase
Protein
Lipids (fats and oils)
Polypeptides
Fatty acids and glycerol
Stomach
Pancreas
Pancreatic amylase
Maltase
Starch
Maltose
Maltose
Glucose
Pancreas
Small intestine
Pancreatic trypsin (Protease)
Chymotrypsin
Carboxpeptidase
Protein
Peptides then into amino acids
Pancreas
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•
•
•
•
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Proteases catalyse the breakdown of proteins into amino acids in the stomach and small
intestine.
Lipases catalyse the breakdown of fats and oils into fatty acids and glycerol in the small
intestine.
Amylase catalyses the breakdown of starch into maltose in the mouth and small intestine.
Maltase catalyses the breakdown of maltose into glucose in the small intestine.
In addition to that, sodium hydrogencarbonate salts are also being produced to neutralize
the acidic chyme and create an alkaline medium. (7.5 pH)
Bile (Gall):
•
•
•
•
•
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It's a greenish yellow alkaline (7-8 pH) watery secretion.
Produced in the liver and passed to the gallbladder.
It is stored in the gallbladder and move through the bile duct into the duodenum.
The enzymes in the small intestine work best in alkaline conditions.
It neutralises the (hydrochloric) acid - providing the alkaline conditions needed in the small
intestine.
It emulsifies (mechanical digestion) fats - providing a larger surface area over which the
lipase enzymes can work, by breaking fats globules into small ones.
Bile salts
•
Bile
Sodium hydrogencarbonate
Bile pigment
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Jejunum:
•
•
Connects the duodenum to the ileum.
Partial digestion of food occurs in it, because of
the enzyme secretion from the cells in the walls of
the small intestine.
Ileum:
•
•
It's where complete digestion and absorption
occur.
The longest part of the small intestine (2-4 m)
•
The villi (one is called a villus) are tiny, fingershaped structures that increase the surface area. They have several important features:
1. Wall just one cell thick - ensures that there is only a short distance for absorption to
happen by diffusion and active transport.
2. Network of blood capillaries - transports glucose and amino acids away from the
small intestine in the blood.
3. Internal structure called a lacteal - transports fatty acids and glycerol away from the
small intestine in the lymph.
•
Brush border enzymes are special enzymes found on the microvilli of the small intestine
that complete digestion.
•
Absorption: The movement of completely digested food molecules and ions into the blood
stream.
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Chemical digestion in ileum:
•
Substrate
Enzyme
End result
Maltose
Maltase
Glucose
Peptides
Peptidase
Amino acids
Emulsified fats
Lipase
Fatty acids
Assimilation: The movement of digested food molecules into the cells of the body where
they are used.
The liver:
•
•
•
•
•
•
The largest gland in the body.
The bile secreting organ.
Glucose is used in respiration to
provide energy.
Amino acids are used to build new
proteins.
In liver:
Glucose
Amino acids
Glycogen (A complex [polysaccharide] carbohydrate used for storage)
Proteins
Deamination: This is the removal of the nitrogen-containing part of amino acids, by the
liver, to form urea, followed by the release of energy from the remainder of the amino
acid.
Functions of the liver: (The 3D, 2SR)
1.
2.
3.
4.
5.
6.
7.
Destruction of old red blood cells.
Detoxification of drugs and alcohol.
Deamination of excess amino acids.
Secretion of bile for fat emulsion.
Regulation of blood glucose level.
Storage of iron and fat soluble vitamins (K,E,D,A)
Regulation of body temperature.
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The large intestine:
Some
Water
•
By the time the contents of the gut reach the end of the small intestine, most digested
food and water has already been absorbed.
•
Except for
Bacteria
(living & dead)
Undigested food (e.g.: cellulose from
plants' cell walls)
Cells (from
the lining
of the gut)
• Functions:
1. The colon is the first part of the large intestine; it
absorbs most of the remaining water, minerals and salts
from the undigested food.
2. This leaves semi-solid waste material called faeces,
which are stored in the rectum, the last part of the large
intestine. Egestion (defecation) happens when these faeces pass out of the body through the
anus.
•
Egestion: The passing out of food that hasn't been digested or absorbed as faeces through
the anus.
Pancreas:
•
To make digestive chemicals (enzymes) which
help us digest food. Enzymes help to speed up
your body's metabolic (biosynthesis) reactions.
•
To make hormones which regulate our
metabolism. Hormones can be released into the
bloodstream. They act as messengers, affecting
cells and tissues in distant parts of your body.
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About 90% of the pancreas is dedicated to making digestive enzymes. Cells called acinar cells
within the pancreas produce these enzymes. The enzymes help to make proteins, fats and
carbohydrates smaller. This helps the guts (intestines) to absorb these nutrients. The acinar cells
also make a liquid which creates the right conditions for pancreatic enzymes to work. This is also
known as pancreatic juice. The enzymes made by the pancreas include:
•
•
•
Pancreatic proteases (such as trypsin and chymotrypsin) - which help to digest proteins.
Pancreatic amylase - which helps to digest sugars (carbohydrates).
Pancreatic lipase - which helps to digest fat.
Approximately 5% of the pancreas makes hormones which help to regulate your body's
metabolism.
•
•
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Insulin - which helps to regulate sugar levels in the blood.
Glucagon - which works with insulin to keep blood sugar levels balanced.
Gastrin - which aids digestion in the stomach.
Types of Pancreatic Enzymes and Their Effects:
Lipase
Effects:
Lipase works with bile from the liver to break down fat molecules so they can be absorbed and
used by the body.
Shortage may cause:
• Lack of needed fats and fat-soluble vitamins.
• Diarrhoea and/or fatty stools.
Protease
Effects:
Proteases break down proteins. They help keep the intestine free of parasites such as bacteria,
yeast and protozoa.
Shortage may cause:
• Allergies or the formation of toxic substances due to incomplete digestion of proteins.
• Increased risk for intestinal infections.
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Amylase
Effects:
Amylase breaks down carbohydrates (starch) into sugars which are more easily absorbed by the
body. This enzyme is also found in saliva.
Shortage may cause:
•
Diarrhoea due to the effects of undigested starch in the colon.
Diarrhoea:
•
•
From the diseases related to the digestive system is Diarrhoea; Diarrhoea is the loss of
watery faeces
The second largest cause of death among children after pneumonia, a person with severe
diarrhoea can lose dangerous amounts of water and salts from their body causing their
organs to stop working.
Causes:
•
Not enough water is absorbed from the faeces.
• By Cholera; Cholera is an epidemic infection that spreads through water and food,
which is contaminated with the bacterium of an infected person.
• Cholera bacterium is found in unhygienic places such as refugee camps.
•
Once the bacterium infects the intestine, it secretes the enterotoxin (a protein exotoxin
that targets the intestines) from its external coating. The enterotoxin binds to a
receptor (an organ which responds to external stimuli) on the cells of the lining of the
small intestine. Part of the toxin then enters the intestinal cells. The toxin increases the
activity of an enzyme that regulates a cellular pumping mechanism that controls the
movement of water and electrolytes between the intestine and the circulatory system.
This pump effectively becomes locked in the “on” position, causing the outflow of
enormous quantities of fluid—up to one litre (about one quart) per hour—into the
intestinal tract. All of the clinical manifestations of cholera can be attributed to the
extreme loss of water and salts.
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Treatment:
•
Treatment consists largely of replacing lost fluid and salts with the oral or intravenous
administration of an alkaline solution of sodium chloride. For oral rehydration the solution
is made by using oral rehydration salts (ORS)—a measured mixture of glucose, sodium
chloride, potassium chloride, and trisodium citrate.
The kidney: Filters initial urine from the blood, reabsorb water and nutrients, and secrete
wastes, producing the final urine, which is expelled.
Blood glucose level:
Normal blood glucose level is 80-120 mg / 100 cc of blood
Q1: When and what happens if the blood glucose level rises above normal?
After eating a meal rich in carbohydrates.
1. Liver regulates blood glucose level.
2. Pancreas secrete insulin hormone.
3. Glucose Insulin Hormone Glycogen (to be stored in liver and muscles)
In pancreas
4. Blood glucose level return to normal.
5. A negative feedback mechanism to stop correction order. (Pancreas stops secreting insulin)
Q2: When and what happens if blood glucose level fall below normal?
When fasting.
1. Liver regulates blood glucose level.
2. Glycogen is broken down by glucagon hormone secreted from pancreas.
3. Glycogen
Glucagon Hormone
In pancreas
Glucose
4. Blood glucose level return to normal.
5. A negative feedback mechanism to stop correction order. (pancreas stops secreting
glucagon)
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Enzyme Name
Aminopeptidase
Source
Small
intestine
Pancreatic
Carboxypeptidase acinar
cells
Substrates
Notes (if
applicable)
Products
Amino acid
at amino
end of
peptides
Amino acids &
peptides
n/a
Amino acid
at carboxyl
end of
peptides
Amino acids &
peptides
Activated from
procarboxypeptidase
by trypsin; optimum
pH varies
Chymotrypsin
Pancreatic
acinar
cells
Proteins
Peptides
Activated from
chymotrypsinogen
by trypsin; optimum
pH 7.8
Gastric lipase
Stomach
chief cells
Triglycerides
Fatty acids &
monoglycerides
Optimum pH 4-5
Lactase
Small
intestine
Lactose
Glucose &
galactose
n/a
Maltase
Small
intestine
Maltose
Glucose
n/a
Pancreatic
amylase
Pancreatic
acinar
cells
Starches
Maltose,
maltotriose, αdextrins
Optimum pH 6.7-7.0
Pancreatic lipase
Pancreatic
acinar
cells
Triglycerides
emulsified
by bile salts
Fatty acids &
monoglycerides
Optimum pH 8.0
Pepsin
Stomach
chief cells
Peptides
Activated from
pepsinogen by
pepsin & HCl;
optimum pH 1.5-1.6
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Proteins
Biology / Chapter 7
Sucrase
Small
intestine
Salivary amylase
Salivary
glands
Trypsin
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Pancreatic
acinar
cells
Sucrose
Glucose &
fructose
n/a
Starches
Maltose,
maltotriose
(trisaccharide),
α-dextrins
Optimum pH 6.7-7
Peptides
Activated from
trypsinogen by
enterokinase;
optimum pH 7.8-8.7
Proteins
Biology / Chapter 7